I have actually seen it used both ways in the past, it can be a little confusing.

Usually it's an i adjective but on occasion you can come across both chiisai and ookii with this problem.

Here is a quote from a yahoo answers on a google search for an example

"Ookina and Chiisana come from the old word, Ookinari and Chisanari that were not adjectives but "adjective verbs".These two forms of that words remain isolated, and work as a noun modifier.Meanwhile "ookii", and "chiisai" are pure adjectives, and have conjugation.There are some idiomatic phrases.大きなお世話だ（This has nothing to do with you.)大きな顔 (Big face)大きな口をきく( speak with big mouse)"